1. Field of the Invention
This invention relates to the production of plastic articles which have the properties of high flexural and impact strength as well as good corrosion resistance.
2. Description of the Prior Art
Because of their comparatively low cost and high strength, fiber reinforced plastics are finding an ever expanding use in the construction of many articles, such as manhole segments, storage tanks, water control gates, weir plates, scum baffles, shower stalls, transportation components, boat hulls, and chairs just to name a few. A serious deficiency of such materials, however, is their poor resistance to corrosion, weathering and abrasion. Conventional methods used for molding fiber reinforced plastics result in the reinforcing fiber being near the outer surface and only covered by a thin layer of resin. This resin layer may be worn away in the course of normal use and the fibers thus become exposed. The fibers then absorb moisture and serve as wicks, thus providing multiple paths for corrosive fluids to penetrate into the structure. This causes a reduction in the physical and mechanical properties of the article.
It has been reported in the literature that the deterioration in properties in fiber reinforced plastic laminates can be substantially reduced by the use of surfacing veils such as glass surfacing mat, dacron, orlon, dynel or nylon. The surfacing veil keeps the laminate reinforcing strands away from the mold surface and helps provide a more resin-rich surface on the molded object. This surface is more resistant to corrosion.
A veiled laminate is illustrated on page 389 of Reinforced Plastics, December 1977. It is stated therein that the most reliable laminate construction for corrosion resistance is the illustrated one, the most important feature of which is the primary corrosion barrier which is rich in resin and is reinforced with "C" glass veil followed by two layers of chopped strand mat. The remaining part of the laminate is built up with appropriate layers of reinforcement required to meet mechanical performance specification and may include, for example, altenate layers of woven rovings and chopped strand mat or continuously wound glass filaments. It is further stated that a resin rich outside surface should be included in situations where spillage may occur or where the exterior environment is also corrosive.
Various ways to form a gel coat over the surface of a structure made from fiber reinforced plastic to prevent the exposure of the fiber to the environment are known in the art. Two such prior art techniques are known as "hand layup process" and "wet mat molding process".
The hand layup process involves the use of a preform outlining the contour of the article to be formed. A layer of a thermosetting resin, such as an epoxy resin or polyester resin, and fiberglass are applied to the work surface of the mold. Squeeges or rollers can be used to work in the reinforcement and remove air. For the necessary thickness, additional layers can be added. The layup can be cured at room temperature or accelerated by oven curing. For high quality surfaces, the mold surface can be sprayed with a gel coat prior to the layup. Other techniques such as vacuum bag, pressure bag, etc. can be used instead of manual layup to smooth the resin-fiber layers and eliminate air. U.S. Pat. Nos. 3,245,865 and 3,257,266 disclose hand layup processes.
The wet mat process is a matched die molding process and involves the formation of an article or composite utilizing matching male and female dies for forming and curing a mat, fabric, or preform into the contour of the finished part. The reinforcement is combined with catalyzed resin at the press prior to or just after placing in the mold. This operation is usually conducted at temperatures of 225.degree.-350.degree. F., pressures of 100-3000 psi. etc., high enough to cure the thermosetting resin component of the mix. For example, a glass mat is cut to the approximate size of the die and is placed onto the die surface either before or after impregnation with a thermosetting resin. A thermosetting or thermoplastic resin sheet cut to the proper size is then placed over the impregnated glass mat. The composite is then pressed between the male and female dies to the contour of the desired article. In this system, the molding operation utilizes relatively low pressures, e.g., up to about 3 k.s.i., and very little, if any, material flow occurs. U.S. Pat. Nos. 3,454,421; 3,616,185; and 3,679,510 disclose wet mat processes.
The March 1976 issue of Plastics Technology, page 41, states that corrosion resistance has been obtained in a variety of SMC (sheet molding compound) parts for sewage-treatment plants and sewer manholes, this being an improvement over the filament winding and hand preforming or layup process traditionally dominant in this field. It is stated that this is accomplished by incorporating a synthetic-fiber surfacing veil to protect the chopped glass from chemical exposure and possible wicking effects; that the fabric or veil is run on one side of the SMC, either under the doctor blade or deposited further downstream, with normal resin paste and chopped glass placed either above or below the cloth. A charge of this material is placed in the mold veil-side down with regular SMC above it to make up the total charge weight. This reference does not disclose the properties which the fabric or veil must possess in order to be satisfactory. The properties of the veil are, however, quite critical in order to obtain the desired results during the molding operation.